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Ubuntu LinuxOperating system · Canonical

CVE-2019-12523

CRITICAL · 9.1 CVSS v3.1 Published 2019-11-26
Fix available
A fix is available. Upgrade to after 4.8 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
An issue was discovered in Squid before 4.9. When handling a URN request, a corresponding HTTP request is made. This HTTP request doesn't go through the access checks that incoming HTTP requests go through. This causes all access checks to be bypassed and allows access to restricted HTTP servers, e.g., an attacker can connect to HTTP servers that only listen on localhost.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

In Squid before 4.9, the URN request handler makes an internal HTTP request to fetch the requested resource, but this internal request completely bypasses all configured access control lists (ACLs) and access checks. This allows any authenticated proxy user to reach restricted HTTP servers (e.g., localhost-only services) that should be protected by the proxy's ACL rules.

MitigationUpgrade Squid to version 4.9 or later to receive the patch that enforces access controls on internally-generated HTTP requests triggered by URN requests.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Ubuntu LinuxOperating system
Affected:= 16.04= 18.04= 19.04= 19.10
FedoraOperating system
Affected:= 30= 31
Debian LinuxOperating system
Affected:= 9.0= 10.0
SquidApplication
Affected:>= 3.0, <= 3.5.28>= 4.0, <= 4.8
LeapOperating system
Affected:= 15.0

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify installed Squid version
    Run `squid -v` or check your package manager (dpkg -l squid, rpm -q squid) to find the exact version number
    Affected if Version is 3.0 through 3.5.28, or 4.0 through 4.8 (vulnerable range)
  2. Locate Squid configuration file
    Common paths are /etc/squid/squid.conf or /etc/squid3/squid.conf; check for custom paths via `squid -k parse 2>&1 | head -20`
    Affected if Configuration file exists and Squid is actively running with it
  3. Verify if URN handler is enabled
    Search the squid.conf file for 'urn:' directives or 'refresh_rewrite' rules referencing URN; also check for 'acl URN' entries. URN support is typically enabled via the 'urn:' protocol handler in the config
    Affected if URN handler is explicitly configured or enabled in the configuration
  4. Identify ACL rules protecting sensitive resources
    Review squid.conf for 'acl' directives and 'http_access' rules, particularly those restricting access to localhost (127.0.0.1), private networks, or specific ports
    Affected if ACL rules exist that should block access to certain resources but URN requests can bypass them
  5. Confirm Squid is accepting URN requests
    Test by attempting a URN request through the proxy (e.g., `curl -U user:pass -x http://proxy:3128/urn:resource`) or review access logs for 'urn:' URL patterns
    Affected if URN requests are being processed by the Squid instance

You are affected if Squid version falls within 3.0-3.5.28 or 4.0-4.8 AND the URN handler is enabled in your configuration, regardless of what ACL rules you have defined.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to a release after 4.8
Interim mitigation

Upgrade Squid to version 4.9 or later to receive the patch that enforces access controls on internally-generated HTTP requests triggered by URN requests.

Recommended fix High confidence

Squid 4.9 or later

  1. Upgrade Squid to version 4.9 or later to resolve the URN request access control bypass vulnerability
  2. After upgrading, verify that Squid starts correctly and test URN functionality to confirm the fix is working
  3. Review Squid access control lists (ACLs) to ensure they are properly configured for HTTP requests

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Ubuntu Linux Scoped from the published advisory
  • Consultation2.0 h
  • Implementation2.0 h
  • Testing4.0 h
  • Review / QA2.0 h
10.0 hours of engineering $1,720
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2019-12523 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data